This study will assess the pharmacokinetic and safety profile of treprostinil following fixed and escalating doses of treprostinil diethanolamine SR tablets. Open-label, two-part study assessing the pharmacokinetics, safety, and tolerability of oral treprostinil diethanolamine SR. Cohort 1: single 1 mg treprostinil diethanolamine SR dose. Cohort 2: escalating doses of treprostinil diethanolamine SR up to a target dose of 4 mg BID.